One of the mamjor threats to surface ships is the surface-skimming type of missile. Currently-employed defense of ships against surface-skimming and other types of anti-ship missiles calls for the complementary employment of both guns and anti-missile missiles. More specifically, the relatively expensive anti-missile missiles are effective at longer ranges. However, for shorter ranges, with their attendant short response time, rapid-fire medium-caliber gun-fired projectiles are preferred. While these projectiles, which may employ proximity sensors to initiate fragmentation, are very inexpensive, they are not guidable after firing and a great number must be used to achieve a probability of target destruction.
A system of using a continuous wave laser beam to control the high explosive detonation acceleration of masses carried by low-cost spin-stabilized projectiles, thereby improving the trajectory of the projectiles, has been developed. A salient advantage of this system is that the receiver is mounted in a shrouded portion of the boattail to prevent radiation other than that from a source behind the projectile from being received. Thus, the system is effectively countermeasure-proof. The structure and operation of this system are described in commonly-assigned U.S. Pat. No. 3,860,199, the teachings of which are hereby incorporated by reference. Foreign patents based on this patent are as follows: Canada: 1,009,370; 1,014,269--Switzerland: 561,893; 574,094--Italy: 976,742--Israel: 41,097; Great Britain: 1,429,941--France: 7300093--Germany: 2264243, 2500232. While the operation of this system is satisfactory, improvements in operating range and accuracy are always desired.
It has also been proposed to lay explosives in helical grooves in the body of a projectile to provide thrust and also a torque thereby reducing low frequency precession and higher frequency nutational motion, so that a body-fixed nose-seeker might be feasible. Nose seekers rely on radiated energy produced or reflected by the target while beam riders are controlled by emitted radiation at or near the gun system. Unfortunately, such helical grooves are expensive and difficult to fabricate. For further information regarding this projectile and its operating system, reference may be made to U.S. Pat. No. 4,347,996. Helical grooves are unnecessary in a beam-riding projectile because the gyroscopic motions due to a small transient yaw produced by the thruster action diminish with an exponential time constant on the order of several tenths of a second, and hence, by proper sequencing of the explosive thrusters, can easily be tolerated.
With the present state of art, a 1.06 micron wavelength Neodymium YAG laser for shipboard use can transmit 200 millijoule pulses of 50 nanoseconds duration at pulse repetition frequencies of about 100 Hertz. Laser rangefinders using such parameters are regularly mounted on, and boresighted with, anti-ship-missile system millimeter radar tracking units to provide more accurate target positions. They are generally used at ranges, varying with visibility, of 3-12 kilometers. These desired trajectories of projectiles to be fired at the target are calculated by fire control computers, employing the most updated information about target position. Nevertheless, after the projectile leaves the gun, trajectory errors accrue due to unpredictable target motion, wind, and the usual projectile dispersion relating to a large number of uncontrolled variables.